A cell-phone and a mobile information terminal each being equipped with an image pickup apparatus are coming into wide use, with a trend of enhancing performance and downsizing of an image pickup apparatus using a solid-state image pickup element such as a CCD (Charged Coupled Device) type image sensor or a CMOS (Complementary Metal Oxide Semiconductor) type image sensor. Furthermore, there are increasing demands for further downsizing for the image pickup lens that is mounted on the image pickup apparatus of this kind.
As an image pickup lens to be used for this purpose, there has been proposed an image pickup lens composed of four elements, because it can provide higher property than a lens composed of two elements or three elements. As an example of the image pickup lens composed of four elements, there has been disclosed a so-called image pickup lens of an inverted Ernostar type that is made up of the first lens having positive refractive power, the second lens having negative refractive power, the third lens having positive refractive power and of the fourth lens having positive refractive power, in this order from the object side, for providing higher property, as described in, for example, Unexamined Japanese Patent Application Publication (JP-A) No. 2004-341013.
As another example of the image pickup lens composed of four elements, there has been proposed a so-called image pickup lens of a telephoto type that is made up of the first lens having positive refractive power, the second lens having negative refractive power, the third lens having positive refractive power and of the fourth lens having negative refractive power in this order from the object side, and is targeted to be shorten the total length of the image pickup lens, as described in, for example, JP-A No. 2002-365530.
The image pickup lens described in JP-A No. 2004-341013 is of an inverted Ernostar type in which the fourth lens is a positive lens. A principal point of the optical system is positioned on the image side and a length of its back focal length is longer, compared with a telephoto type lens in which the fourth lens is a negative lens. Therefore, this type of image pickup lens is disadvantageous to be downsized. In addition, there is one lens with a negative refractive power out of the four lenses in the optical system, which makes the correction of Petzval's sum difficult, resulting in a problem that excellent performance is hardly secured on a peripheral portion of an image. Further, in the image pickup lens described in JP-A No. 2002-365530, its angle of view is narrow and aberration correction is insufficient. It causes a problem that the image pickup lens hardly copes with a higher-pixel image pickup element due to the deterioration of its optical property when the total lens length is further shortened.